Astronomy Exam 3

Earth's layers (outside to in)

Crust, mantle (hot solid rock that slowly convects), liquid outer core (iron/nickel), solid inner core (iron/nickel)

Differentiation

Dense material sinks to the center and lighter material rises, forming layers (core, mantle, crust)

P waves (pressure)

Compressional seismic waves; travel through solids AND liquids; faster

S waves (shear)

Seismic waves that travel only through solids; slower; blocked by Earth's liquid outer core

How do we know Earth's outer core is liquid?

S waves are blocked by it (shadow zone) and P waves bend as they pass through

What causes Earth's magnetic field?

A dynamo: convection of the liquid metallic outer core plus Earth's rotation

Earth's magnetic field characteristics

Roughly dipolar, tilted slightly from the rotation axis, reverses occasionally; forms the magnetosphere that deflects the solar wind

Aurora

Charged solar-wind particles funneled by the magnetic field toward the poles hit atmospheric oxygen and nitrogen, making them glow; more frequent near sunspot maximum and after CMEs

What does a planetary magnetic field imply?

A liquid, conducting, convecting interior plus rotation

Plate tectonics

Lithosphere is broken into plates that move a few cm/year, driven by mantle convection

Rift (divergent) zone

Plates move apart and new crust forms (mid-ocean ridges, East African Rift)

Subduction (convergent) zone

One plate dives beneath another, producing trenches, volcanoes, and earthquakes; colliding continents make mountains

Transform boundary

Plates slide past each other (San Andreas Fault)

Continental drift (Wegener)

Continents were once joined (Pangaea); evidence: matching coastlines, fossils, and rock types across oceans

Where is volcanic activity on Earth concentrated?

Mostly along the borders of tectonic plates

Atmosphere layers (bottom to top)

Troposphere (weather), stratosphere (ozone), mesosphere, thermosphere (auroras), exosphere

Major constituents of Earth's atmosphere

About 78% nitrogen, 21% oxygen, about 1% argon, trace CO2 and water vapor

Greenhouse effect

Surface re-radiates infrared; gases like CO2, water vapor, and methane absorb it and re-emit it downward, warming the planet

Why is the greenhouse effect important?

Without it Earth would be well below freezing; it makes life possible

Why are fossil fuels a concern?

Burning them adds CO2 that strengthens the greenhouse effect and drives global warming

Major driver of global climate change

Buildup of CO2 and other greenhouse gases (NOT ozone)

Other planet with an extreme greenhouse effect

Venus (runaway greenhouse effect)

Ozone layer

Stratospheric O3 that absorbs harmful ultraviolet light

CFCs

Refrigerant/aerosol chemicals that release chlorine, destroying ozone (the ozone hole); banned by the Montreal Protocol

Lunar maria

Dark smooth plains; ancient lowland basins filled with lava billions of years ago (not oceans)

Lunar highlands

Bright, old, heavily cratered terrain

Regolith (lunar dust)

Fine powdery soil produced by billions of years of impacts breaking up surface rock

Why does the Moon preserve so many craters?

No atmosphere, no weather or erosion, and little geologic activity

Near side vs. far side of the Moon

Near side has large maria; far side is mostly heavily cratered highlands because its crust is thicker so lava rarely reached the surface; both get equal sunlight

Does the Moon have an atmosphere?

Essentially none (too little gravity)

Does the Moon have a magnetic field?

None today (small, mostly solid core)

Why do we always see the same side of the Moon?

Synchronous rotation: it rotates once per orbit

Moon vs. Earth density

Moon is less dense (small iron core)

Water ice on the Moon

Found in permanently shadowed polar craters, probably delivered by comets; does NOT mean liquid water once covered the surface

Co-accretion theory (Moon)

Earth and Moon formed together; problem: Moon lacks an iron core and has a different density/composition

Capture theory (Moon)

Moon formed elsewhere and was captured; problem: hard gravitationally, and Moon rocks match Earth's mantle isotopically

Fission theory (Moon)

Fast-spinning young Earth flung off the Moon; problem: Earth could not spin fast enough and angular momentum is wrong

Giant impact (collision-ejection) theory

A Mars-sized body (Theia) hit the young Earth and the debris formed the Moon; the currently accepted theory

Why is the giant impact theory accepted?

Explains the Moon's small iron core and low density, rocks matching Earth's mantle, early molten surface, and the system's angular momentum

Terrestrial planets

Mercury, Venus, Earth, Mars

Terrestrial planets by size (largest to smallest)

Earth, Venus, Mars, Mercury

Smallest planet

Mercury

Densest planet

Earth

Hottest planet

Venus (about 460 C, runaway greenhouse)

Which terrestrial planets have moons?

Earth (1) and Mars (2: Phobos and Deimos); Mercury and Venus have none

Which terrestrial planets rotate slowly?

Mercury (3 rotations per 2 orbits) and Venus

Which planet rotates backward (retrograde)?

Venus (very slowly; its day is longer than its year)

Mercury features

Cratered like the Moon, little or no maria, almost no atmosphere, weak magnetic field, scarps from planetary shrinking

Scarps on Mercury

Cliffs showing the planet shrank as its core cooled long after formation

Venus atmosphere

Very thick CO2 (about 90x Earth's pressure) with sulfuric acid clouds

Venus surface age

Only a few hundred million years old (from crater counts); global resurfacing by volcanism

Why did Magellan use radar at Venus?

Thick clouds block visible light; radar passes through them

Does Venus have a magnetic field?

No (no strong global field)

Mars atmosphere

Thin, low-density CO2; low pressure; temperatures vary with seasons

Olympus Mons

Tallest volcano/mountain in the solar system; on Mars

Valles Marineris

Huge canyon system on Mars

Percival Lowell's Martian canals

An optical illusion

How do we know some meteorites came from Mars?

Gases trapped in the rock match Mars's atmosphere

Which planets could possibly sustain life?

Earth (certainly) and Mars (possibly past or microbial); Venus is too hot

Why is Mars better than Venus for colonization?

Moderate temperatures, thin atmosphere (not crushing), water ice, Earth-like day, solid surface; needs thicker atmosphere, oxygen, radiation protection, and water

Jovian (giant) planets

Jupiter, Saturn, Uranus, Neptune

Largest planet

Jupiter

Smallest giant planet

Neptune (slightly smaller than Uranus in diameter)

Least dense planet

Saturn (would float)

Which planet has an extremely tilted axis (about 98 degrees)?

Uranus (extreme seasons)

Which giants have liquid metallic hydrogen?

Jupiter and Saturn; it produces their strong magnetic fields

Ice giants

Uranus and Neptune

Giant planet atmospheres are dominated by

Hydrogen (then helium), plus methane, ammonia, and water compounds

Giant planet storms

Larger and longer-lived than on terrestrials (Great Red Spot, Great Dark Spot)

Internal heat of giant planets

Jupiter, Saturn, and Neptune radiate as much or more energy from internal convection as they get from the Sun

Planet with the longest orbit

Neptune (not Uranus)

Which planets have rings?

All four giants; Saturn's are the most spectacular

Saturn's rings are made of

Billions of small objects in orbit, made of ice and rock

Voyager 2

Flew past Uranus (1986) and Neptune (1989); never landed

Io

Jupiter's moon; most volcanically active body; active lava volcanoes powered by tidal heating

Europa

Jupiter's moon; ice crust over a subsurface liquid water ocean; top candidate for life

Ganymede

Jupiter's moon; largest moon in the solar system; own magnetic field; probable subsurface ocean

Callisto

Jupiter's moon; heavily cratered ancient surface; possible subsurface ocean

Titan

Saturn's moon; thick nitrogen atmosphere, methane/ethane lakes and seas; Huygens landed here

Enceladus

Saturn's moon; water geysers from the south pole; subsurface ocean; candidate for life

Triton

Neptune's moon; retrograde orbit (probably captured Kuiper Belt object); nitrogen geysers

Which moons might support life?

Europa and Ganymede (Jupiter), Enceladus and Titan (Saturn); liquid water or hydrocarbons, energy (tidal heating), organic chemistry

Exoplanet detection methods

Transit (star dims) and radial velocity (stellar wobble); also direct imaging and microlensing

Hot Jupiters

Giant planets very close to their stars; explained by planetary migration

Crater counting

More craters means an older surface; gives relative ages

Radioactive dating

Uses decay of radioactive isotopes to give absolute ages

Age of the solar system

About 4.5 billion years, from radioactive dating of primitive meteorites

Solar nebula theory step 1

A cloud of gas and dust (solar nebula) collapses under gravity

Conservation of angular momentum (nebula)

As the cloud shrinks it spins faster and flattens into a disk

Protosun

Dense hot center of the nebula that begins fusion and becomes the Sun

Condensation

Solids form from cooling gas; only metal and rock inside the frost line, ices too beyond it

Accretion

Early process in which colliding particles stick together and grow

Planetesimals

Small bodies formed by accretion that merge into protoplanets

Why do giant planets have H and He?

They grew large (aided by ices beyond the frost line) and could capture gas; terrestrials could not

Why did terrestrial planets form close to the Sun?

Only metal and rock could condense there; these are scarce so the planets stayed small (NOT because rock was more plentiful)

Evidence for the solar nebula theory

Coplanar same-direction orbits, same-direction spins, composition trends, disks around young stars, meteorite ages

Problems for the solar nebula theory

Hot Jupiters, Uranus's tilt, Venus's retrograde rotation, origin of the Moon

Why are Earth-like worlds differentiated?

Continuing impacts on a growing protoplanet melted the entire body

Which solid worlds stay geologically active longest?

The largest (they retain heat best)

How was the Sun's distance determined?

Radar ranging and Kepler's laws (the Astronomical Unit)